Flow cytometry is a major tool for basic research in immunology, molecular and cell biology, and genetics as well as in clinical applications. It has been widely used by immunologists to investigate lymphocyte development and purify lymphocytes for biochemical and functional studies, and by cell biologists to study calcium mobilization and cell cycle. With the advancement of new reporter genes such as green fluorescent protein (GFP), and with its ability to select and purify rare mutant cells, flow cytometry has become a major research tool for molecular biologists and geneticists investigating gene expression and function. Findings as revealed by flow cytometers have advanced our understanding in basic biological processes as well as in cancer and infectious diseases. The Center for Cancer Research at the Massachusetts Institute of Technology has been operating a shared Flow Cytometry Core Facility since 1987, serving the entire MIT research community, Whitehead Institute for Biomedical Research, and Woods Hole Oceanographic Institute, on first come first served basis. Because of the increased demand, our facility is no longer adequate to serve the increasing numbers of users, as indicated by a two to three week waiting period for cell sorting experiments. Furthermore, our facility is not able to meet the demand by more and more investigators whose research requires flow cytometers with the capacity to analyze more than 6 colors simultaneously and to sort cells at more than 20,000 cells/second. In addition, one of our major flow cytometers (FACStar Plus) is over 10 years old and is becoming out-dated and is losing support from the manufacturer. To meet the needs of the research community, our shared Flow Cytometry Core Facility requires a new high speed cell sorter capable of sorting 20,000 cells per second and of analyzing 6-colors at the same time as requested in this application.